The relevance of inertia-capillary dynamics on superhydrophobic heat exchange, microscopic phase change, and bloodstains
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For most surfaces, an impacting water drop will stick upon contact; however, for a superhydrophobic surface, an impacting water drop can spread out and recoil to such an extreme that it can completely bounce off of the surface. The duration for which a drop is in contact with the surface depends on a balance between inertial and capillary effects, leading to a contact time often on the order of milliseconds. Previous studies have focused on the dynamics of the drop interface during this short time; however, it is unclear in what ways the inertia-capillary dynamics interact with transport phenomena across the interface that might occur on similar timescales. Here we combine modeling and experiments to investigate the relevance of the inertia-capillary dynamics on three interfacial transport processes: superhydrophobic heat exchange, microscopic phase change, and on the size and shape of dried bloodstains. Our first study focuses on heat exchange between a bouncing drop and a superhydrophobic substrate. By measuring the thermal interaction between a superhydrophobic substrate and a heated or cooled drop, we demonstrate that the contact time is short enough that only a small fraction of potential heat is transferred, and, counter-intuitively, smaller drops transfer a larger fraction of their potential heat than larger drops despite contacting the surface for less time. Our results indicate that birds with superhydrophobic feathers will be warmer in cold rain than those with feathers on which drops stick, and we envision that a better understanding of these mechanisms can inspire the design of novel superhydrophobic materials to control heat exchange. Our next study focuses on certain superhydrophobic surfaces that a water drop will stick on them rather than bounce if it is sufficiently hot. We model two potential mechanisms in which a superhydrophobic surface could trap a sufficiently hot drop within milliseconds: a microtexture melting mechanism and an evaporation–condensation mechanism. Ultimately, we aim to address how one might design a smart superhydrophobic surface in which the surface can sense a property of the drop, here its temperature, and, if above a critical threshold, passively adjust its functionality so that it will capture the drop and act as a rapid thermal fuse. Our last study focuses on how microscopic coatings can modify bloodstain shapes and sizes with the goal of challenging some tacit assumptions in forensics. We demonstrate that the inertia-capillary dynamics sets the size of bloodstains on different coatings, including on unintentional coatings, such as the sebaceous residue from a latent fingerprint. Since the stain size and shape can be critical in conducting a bloodstain pattern analysis, our results highlight the need for forensic analysts to exercise caution when evaluating bloodstains on surfaces that might contain coatings or residues.